1. Field of the Invention
Conducting polymers have been used as protective coatings. Although these types of polymers appear to be effective in most cases, problems have resulted when attempting to bind these polymers to an aluminum surface. The polymers of this invention adhere extremely well to aluminum and thus, provide a very effective form of corrosion protection.
2. Description of the Prior Art
Conducting polymers are used as replacements for metals because the conducting polymers have potentially unique and/or superior properties compared to anodization and chrome conversion coatings. Furthermore, and of ever-increasing importance, some metals are toxic and in numerous instances have found to cause considerable damage to the environment. Therefore, the discharge of these substances is being closely monitored if not banished entirely.
There are several proposed mechanisms for corrosion protection, one or more of which could be occurring at any time. The first is a simple galvanic process by which the polymer has a lower oxidation potential than the metal it is protecting resulting in the preferential oxidation of the polymer. Since oxidized polymers are usually insoluble and do not dissolve away as zinc does, corrosion protection with conducting polymers lasts longer than a conventional galvanic coating such as zinc.
Another proposed mechanism for corrosion protection occurs when the polymer reacts with the surface of the metal, requiring that the polymer have an oxidation potential higher than that of the metal. The surface of the metal reacts with the polymer and forms a passivating layer which inhibits further corrosion by either setting up a barrier or by changing the surface potential or both.
Current methods of corrosion protection, i.e. zinc, do not last very long (usually less than 2 years) and/or are coming under increased scrutiny by the Environmental Protection Agency. For example, the use of chromium and cadmium for anti-corrosion coatings will soon be banned. Current mechanisms for corrosion protection include the use of a sacrificial electrode, such as a zinc coating, which will corrode (oxidize) in the place of the substrate. One major limitation in utilizing a sacrificial electrode is that the coatings do not last very long. The oxidized zinc metal dissolves or easily becomes scaled when subjected to water or moisture. Therefore, environmental concerns arise since toxic metals are being released.
Barrier coatings such as epoxy are currently being employed, but are not very robust especially when a pit in the coating has been formed. Once a pit forms, the corrosive species attacks the underlying metal and, thereby, increases the exposed surface, accelerating the corrosion process.
The corrosion inhibiting properties of conducting polymers was suggested by MacDiarmid in 1985 (MacDiarmid, A. G., "Short Course on Electrically Conductive Polymers", New Paltz, N.Y., 1985). Initial studies on the protection of metal surfaces against corrosion by conducting polymers was reported in the literature that same year (Ref: DeBerry, D. W., J. Electrochem. Soc, 132, 1022, (1985)). Much of the work on corrosion protection has focused on polyaniline (PANI), but also has been extended to other conjugated polymers.
Corrosion occurs by oxidation of a metallic surface by a medium to produce oxides and hydroxides. As these form, soluble species are produced, the surface pits increasing its surface area and the rate of decomposition accelerates until the surface is completely covered with scale or is totally corroded. As mentioned previously, one way to provide corrosion protection is to coat the metal with a barrier to prevent the reactive species from reaching the surface. Galvanization with zinc (or other metal with low enough oxidation potential) prevents corrosion via the creation of an interfacial potential at the metal:zinc interface. The zinc will corrode preferentially, while the reactive species may come in contact with the metal, the increased oxidation potential causes the metal to be unreactive, thereby, inhibiting the corrosion process.
Prior studies utilizing PANI as a corrosion protection coating shows that it works quite well. In fact, exposed metal surfaces adjacent to conducting polymer coatings (scratches or edges) are unreactive to corrosion as reported by Thompson and coworkers (refs: Baughman, R. H. et. al. Journal of Chemical Society, Chem. Comm. 49, 1977 and Nowak, R. J. et.al., Journal Chemical Society, Chem. Comm. Comm. 9, 1977). The corrosion protection properties of PANI on aluminum have also been studied. There has been some success in showing that PANI provides corrosion protection in the short term i.e. 7 days. R. Racicot, R. L. Clark, H-B. Liu, S. C. Yang, Thin Film Conductive Polymers On Aluminum Surfaces: Interfacial Charge-Transfer and Anti-Corrosion Aspects, SPIE Proceedings, Optical and Photonic Applications of Electroactive and Conducting Polymers, Vol. 2528, Jul. 12-13, 1995 San Diego, Calif. However, in the long term, 1-6 months, PANI has been found to not adhere well to aluminum surfaces.
One disadvantage in utilizing PANI is that the corrosion protection ability is pH dependent. In acidic media, PANI coated mild steel coupons corrode 100 times slower than uncoated counterparts, while in pH 7 media, the PANI coated material corrodes 2 times slower (Ref: Lu, W-K, Elsenbaumer, R. L., Wessling, B., Synthetic Materials, 71, 2163 (1995)) and Wei, Y., Wang, J., Jia, X., Yeh, J-M., Spellane, P., Polymer 36(23), 4536 (1995)). This could be explained by the pH dependence of the structure of PANI. At low pH, the conducting emeraldine salt is the predominant form. At high pH, the non-conducting emeraldine base is the predominant form. It appears that the conducting form is required for the formation of the passivation layer.
In summary, the effectiveness of the corrosion protection is controlled by the type of polyaniline (emeraldine base versus emeraldine salt), the characteristics of the corrosion environment (acidic medium, aqueous sodium chloride, etc.) and by adhesion to the substrate. For optimum corrosion protection, it may be necessary to develop conducting polymers that do not have the pH dependence of conductivity that PANI has.
At present, some conducting polymers (in their neutral, non-conducting states) are soluble in organic solvents. Various types of surfactant counter-ions have been used with PANI to make the conducting form of polyaniline soluble in organic solvents. The results of this test are ongoing due to adhesion problems between the polymers and aluminum.